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Fishy Gene Hints at How Limbs Evolved From Fins

Two genes found only in fish may be a key piece in the puzzling evolution of limbs.

The genes’ removal from zebra-fish embryos resulted in the loss of actinotrichia — a basic fin component — and made their proto-fins resemble appendages seen in ancient fossils of the first four-legged creatures.

“The loss of actinotrichia may have contributed to the evolutionary transition from fin to limb,” wrote researchers led by University of Ottawa biologists Jing Zhang and Marie-Andrée Akimenko in a study published June 23 in Nature.

During early embryonic development, fins and limbs look strikingly alike. In fish, however, some cells form a pattern of fine fibers. These are the actinotrichia, which form the scaffold on which fin rays are assembled.

In their study, Zhang and Akimenko noticed that two genes, actinodin 1 and 2, are especially active during zebra-fish fin development. These proved to code for previously unknown proteins that mix with collagen to form actinotrichia.

Subsequent searches of animal-genome databases found the actinodin genes in other bony fishes (including whale sharks, living fossils little changed in the 400 million years since the Devonian, before limbs evolved) but not in mammals, birds or amphibians.

When the researchers knocked actinodin genes out of zebra-fish embryos, actinotrichia didn’t form in the resulting fishes’ pectoral fins. Their tails, however, were unaffected. That fits with the evolutionary narrative suggested by fossils of the earliest known four-limbed creatures, which kept their fishy tails even as legs started to form.

Similar general patterns of gene expression are also found in embryonic chickens and mice with extra toes, a condition known as polydactyly.

“This is also in agreement with the fossil record, which indicates that the earliest primitive aquatic tetrapods of the late Devonian were polydactylous,” wrote the researchers.